Low-cost, scalable energy storage systems are needed to improve the energy efficiency of the electrical grid (e.g., load-leveling, frequency regulation) and to facilitate the large-scale penetration of renewable energy resources (e.g., wind, solar). While alternative energy technologies exist, they cannot be directly connected to the grid because of their variable output. Electrochemical energy storage may provide the best combination of efficiency, cost, and flexibility to enable these applications. Of particular interest are redox flow batteries, which are rechargeable electrochemical energy storage devices that utilize the oxidation and reduction of two soluble electroactive species for charging (absorbing energy) and discharging (delivering energy). Unlike conventional secondary batteries, the energy-bearing species are not stored within an electrode structure but in separate liquid reservoirs and pumped to and from the power converting device when energy is being transferred. Because of this key difference, flow battery systems can be more durable than conventional battery systems as electrode reactions are not accompanied by morphological changes due to the insertion or removal of the active species and can be more scalable than conventional battery systems as the energy capacity may be easily and inexpensively modulated by varying the reservoir volume or the species concentration, without sacrificing power density. Thus, while flow batteries may not compete with compact lithium (Li)-ion batteries for portable applications (e.g., cell phones, laptops) due to lower overall energy densities, they are well-suited for large-scale stationary applications.
Since their inception in the 1960s, a large number of aqueous redox flow batteries have been developed including iron-chromium, bromine-polysulfide, vanadium-bromine, and all-vanadium systems. Several aqueous hybrid systems also have been developed, where one or both electrode reactions are a deposition/dissolution process, such as zinc-bromine and soluble lead-acid systems. Organic redox materials also have been utilized, however, organic materials often suffer from stability issues (e.g., due to side reactions), low solubility, and the like. In the case of catholytes, oxidation potentials are often low (e.g., <0.8 V versus SHE).
All current aqueous flow battery designs have functional or cost-performance limitations that hamper large scale adoption of this technology. Thus, there is an ongoing need for new redox flow batteries. The present invention addresses this need.